Tilt-mirror switch arrays are becoming of increasing interest in systems that use optical beams either for transmission of information or for its control.
The most common form of tilt-mirror in such arrays includes a substrate of which the top surface is mirrored to be highly reflective and the back surface is conductive to serve as an electrostatic plate. The substrate is suspended so that its center is supported on a fulcrum about which the substrate can pivot. Pairs of electrodes positioned on opposite sides of the fulcrum are used to create electrostatic forces that pivot the mirror between two stable positions, such that an incident beam can be reflected into a selected one of two different directions, depending on the voltage applied. By applying a control voltage to a selected pair of electrodes to have it attract by electrostatic forces the associated half of the substrate, the mirror can be tilted between the two reflective states. A major problem with such mirrors is the tendency of the mirrors, which are minute is size, to curl, which affects both the direction in which the incident beam is reflected and the optical quality of the reflected beam.
Another form of micromirror for use as a variable reflector in mirror arrays that is of current interest is one that involves a change in attenuation of an incident optical beam rather than a change in the direction of its reflection. Such a mirror is typically formed as a quarter-wave dielectric layer of a material, such as silicon nitride, and supported to act normally as a reflective mirror. Such a mirror is symmetrically suspended over a conductive substrate, typically of doped silicon, by a fixed 3/4 wavelength dielectric spacer, typically of a phosphosilicate glass (PSG). An electrode partially covers the membrane, leaving uncovered but surrounding a coated central portion that serves as the mirror. A voltage applied between the electrode and the underlying substrate creates an electrostatic force that, until eliminated, attracts the membrane symmetrically closer to the substrate. The membrane tension provides a linear restoring force when the electrostatic force is eliminated. When the membrane gap is reduced to about a half wavelength by the electrostatic force, the layer becomes an essentially antireflective coating with close to zero reflectivity. The typically 0.4 micrometer vertical deflection of the central portion is small compared to the typically 200-500 micrometer wide membrane. Mechanically, the device moves by elastic deformation, similar to a tuning fork. Electrically, the device behaves as a tiny capacitor with essentially zero-static power dissipation regardless of the reflectivity state.
A more detailed description of such a device is found in our prior paper entitled, "Dynamic Spectral Power Equalization Using Micro-Optic Mechanics," IEEE Photonics Technology Letters, Vol. 10, No. 10, October 1998, pps. 1040-1042. In this device, the mirror coating to define the mirror area is centrally located on the membrane, and largely surrounded by an electrode so that the electrostatic force acting is relatively uniform over the surface of the membrane. The change in spacing between the mirror coating and the substrate is relatively uniform over the entire area of the mirror coating so that there is little tilt in the mirror area. In this prior paper, we describe a wavelength division multiplexer equalizer that utilizes such a device. Such an equalizer depends primarily on control of the attenuation of the incident light.
There are other mirror applications in which, instead of attenuation, deflecting or steering of the incident light is desired. To that end, it is desirable that the incident light be controllably steered, as by tilting by a prescribed amount, the mirror area on which the light is incident for reflection and possible redirection along a desired path with little attenuation. The present invention is primarily directed at a mirror for use in steering an incident beam.